It could still be a good passenger car though.  I have a bit of a personal barrier to their reliability though having seen two broken down (new) Mazdas being pushed around as a kid and a Miata driving backwards down the street because it was stuck in reverse.  Was a long time ago though.

Mazdas historically don't do well in snow states - major issues with rust. Other than that they tend to hold up fairly well. As a brand they came in 11th place in JD Power's latest VDS, just above the industry average. Amazingly, Mercedes came in 5th, beating out Honda/Acura. There's been an absolutely incredible turn around at M-B. Less than 10 years ago, there were constant stories of SLs with tops that got stuck halfway open, broken ABC systems, air suspensions, etc etc etc. Good for them.

Not so good, Land Rover. Absolute bottom of the barrel once again. Wamp wamp.

Personally I'd go for the Mazda cave so I could go aftermarket though OEs are making that a dying market as is evident on the newer integrated systems.  People with my perspective are about as rare as people that build their own PCs so....

I get the feeling it might be best if you just get the standard factory equipment unless you are mechanically and electrically inclined, otherwise it could get pricey w/ complications.
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Nah it's a turbo which makes making power a lot simpler.  For NA it's much more intensive because you don't have a compressor ramming air into the inake and down to the cylinder.  BMW http://en.wikipedia.org/wiki/BMW_M12 and Toyota had stock blocks in the 80's and 90's capable of making 1,000 hp on a dyno just by adding a turbo and dumping in more fuel.  Boost is basically cheating for engineers unless you are flying and have to worry about altitude and atmospheric pressure where it become necessary or have a restriction on engine size versus power reqt like ships.

For strength all you need is a strong crank (most stock Japanese cranks used to be forged steel and nearly perfectly balanced, even in my old 90's Corolla), a block that won't explode (most old blocks were cast iron, check), maybe some port and valve work on the cylinder heads and race spec'd valve springs to handle higher sustained RPMs w/o valve train float, and new Cams and lobes w/ the appropriate lift and duration for the RPM range you expect to operate at.  Stock fuel injectors can usually do 400hp believe it or not, perhaps a new fuel rail and pump to get the gas flowing.  New exhaust manifold and downpipe to match the exhaust with the gain from the intake.

There's a reason you had 1,000hp Supras and Skylines from the 90s.  A lot of their stock internals were built like tanks.  A standard Corolla engine from the 80's and 90's was built to much higher specification and tighter tolerances than a typical Corvette engine from 5 years ago and likely exceeds current ones in certain areas.  I have a 90's stock JDM 1.6L short block Toyota engine from Japan that was modeled after a F1 Cosworth with heads engineered by Yamaha using 5 valves per cylinder and individual throttle bodies.  Still looking for the right vintage Toyo for it.